Why do solvents with low dielectric constants favor SN2 reactions?
Solvents with low dielectric constants did not interfere the nucleophilic's attack, while the high dielectric constants solvent did it.
The mechanism of SN2 reaction occurs concerted. So, the nucleophilicity is important. The polar solvent (high dielectric constants) solve (as solvent) nucleophile with electropositive charge. This causes the nucleophilicity decreases.